Slag is the main by-product of iron and steelmaking process. Ladle slag, melted fully after secondary refining, usually contains rich CaO, Al2O3 and 2CaO·SiO2 phase, which has a potential value to recycle into steelmaking process as the substitution of lime. However, reuse of ladle slag is easy to induce an accumulation effect of sulphur or phosphorus. In this present work, CaS powder was used to replace the sulphur containing phase in ladle slag to investigate the roasting process under different atmosphere conditions. The influence of temperature and oxygen pressure on sulphur removal mechanism and products composition has been discussed. By adding solid sulfur into molten ladle slag, the results of SEM has showed that sulphur element in ladle slag existed as independent CaS phase. Roasting experiments showed that lower oxygen pressure and higher reaction temperature were beneficial for CaS changing into CaO, it was very important to design the optimum roasting route for sulphur removal from the ladle slag.